Measurement of
link delay asymmetry
802.1 ASbt, 201407 IEEE 802 plenary
Lu Huang, China Mobile ([email protected])
Liuyan Han, China Mobile ([email protected])
Requirement
• Sync accuracy requirement in TD-SCDMA/TD-LTE
- Base stations need frequency sync: +/- 0.05ppm, and phase sync: +/- 3us
- Time sync between NodeB/eNB and Reference clock: +/- 1.5us
- Considering RNC and NodeB will introduce time offset, backhaul network (PTN
for China Mobile) need more precise time synchronization: +/- 1us
• Requirement for link delay asymmetry
- The transport delay of optical fiber is 5us per 1km, so 100 meters
length difference will introduce 250ns error
- In our backhaul network, Some of physical lines had serious
asymmetry, whose error was even up to 6us
•
China Mobile has large backhaul network, compensation for asymmetry of
physical line is really a mandatory requirement
•
This problem is really slowing down the large scale deployment of 1588
Possible solution
• Manual/automatic Line-swapping for error calculation
Master
P TX
Device
o
(PTP master) r
t RX
Fiber line 1
Fiber line 2
RX P
o
r
TX t
t1
Slave
Delay1
t2
Device
(PTP slave)
t3
Delay2
t4
Exchange the
two lines
P TX
Device
o
(PTP master) r
t RX
Fiber line 2
Fiber line 1
RX P
o
r
TX t
t5
Delay2
t6
Device
(PTP slave)
Delay1
t7
t8
Delay1=(t2+t8-t1-t7)/2
Delay2=(t4+t6-t3-t5)/2
•
In China Mobile’s PTN network, only one vendor can support manual lineswapping to calculate the error
•
The status (before or after line-swapping) is informed to the PTN equipments by
the network management system
Suggestions for IEEE 802.1ASbt (1)
If the device or modules support automatic line-swapping
• In order to ensure interoperability, the management parameters
and negotiation mechanism should be standardized
• Define the management parameters for the asymmetry error calculation
• Define the negotiation mechanism to help upstream node and
downstream node do line-swapping at the same time and know whether
the undergoing recording is before or after line-swapping
• There may be two ways to add a negotiation mechanism in the
PTP packets to trigger line-swapping on both sides and
announce the status to each other(before or after line-swapping)
• In the PTP header, there are messageType and Reserved bits which can
be utilized
• In the PTP frame TLV. The TLV could be inserted after the 1588 payload
and used to distinguish the packets before or after line-swapping
• Based on the data before and after line-swapping, calculate delay
asymmetry and record it for the future time synchronization
Suggestions for IEEE 802.1ASbt (2)
If only manual line-swapping
• Management system should be able to controll the slave to start
asymmetry error compensation and inform the status (before or
after line-swapping) to devices
• Define the management parameters for the asymmetry error calculation
• Define the management parameters for the status of line-swapping
• Based on the data before and after line-swapping, calculate delay
asymmetry and record it for the future time synchronization
Other consideration
• Detecting line asymmetry quickly in some specific
topology, e.g. ring topology
• Choose ACC3, which has a PTP
passive port based on default BMC
algorithm, to detect line asymmetry of
the ring
Referenc
e clock
AGG1
AGG2
ACC
1
ACC
5
ACC
2
ACC
3
ACC
4
• ACC3 could run 1588 with upstream
nodes through different paths (different
directions)
• Based on the synchronization
results of different paths, it can find
whether the whole ring is asymmetry
or not
•
When it’s ring topology, you can quickly discover the asymmetry to decide
whether a compensation is necessary
•
Maybe, we can define some similar mechanism in ASbt to detect line asymmetry
before doing compensation
Summury
• Compensation for line asymmetry is really a
mandatory requirement
• Now it seems that line swapping is a relatively simple
way to calculate asymmetry error, although no
automatic line-swapping device exists
• In ASbt, it’s better to define some parameters or steps
to manage the whole process of line-swapping
• Maybe, we could define some simple mechanisms in
ASbt to detect line asymmetry quickly before doing
compensation
Thank you
Q&A